The present invention relates to a display device and a manufacturing method thereof, and, more particularly, to a display device using an oxide semiconductor and a manufacturing method thereof.
In general, amorphous silicon thin film transistors (hereinafter referred to as amorphous Si-TFTs; Thin-Film-Transistors) are used as driving elements for the liquid crystal panel. The amorphous Si-TFT has excellent switching characteristics. The liquid crystal panel driven thereby has a wide variety of uses in a range from small-sized displays (mobile phones or digital cameras) to large-sized liquid crystal displays for televisions of over 30 inches. The amorphous Si-TFT has a low level of mobility as an index representing the movement of electrons, specifically, approximately 0.5 cm2/Vs. On the contrary, its manufacturing processes are relatively short, and it can be manufactured for a large-sized substrate. This enables the wide variety of uses, from the small-sized to large-sized displays.
The inventors of the present application have developed an oxide TFT using an oxide semiconductor layer, as a high performance thin film transistor (TFT) having higher mobility than that of the amorphous Si-TFT, and have developed processes for manufacturing the oxide TFT in accordance with substantially the same processes as those for the amorphous Si-TFT. The oxide TFT has a channel protective layer on the upper part of the semiconductor layer, resulting in high mobility. This realizes a high reliable TFT.
The important subject in the oxide TFT is realization of the high reliable TFT. A problem may occur that the degradation of the TFT accelerates upon entrance of external light toward the oxide semiconductor layer. That is, a problem is that a variation in the threshold voltage due to a voltage stress to the TFT accelerates upon light irradiation toward the oxide semiconductor layer. This variation in the threshold voltage may cause a trouble in image display, when it happens in the TFTs applied in pixels or TFTs applied in peripheral circuits.
Japanese Unexamined Patent Publication No. 2010-161373 discloses a technique for forming a protective layer including titanium oxide or titanium oxynitride, using a direct current reactive sputtering method. When an attempt is made to form the protective film using this sputtering method, it needs to have a thickness sufficiently enough to effectively function, thus causing a problem of decreasing manufacture throughput.
Japanese Unexamined Patent Publication No. 2010-283326 discloses a technique for forming oxide titanium, by performing an oxidation process, after formation of a pattern including titanium or titanium alloys on the upper part of the oxide semiconductor layer. When to form a titanium oxide film on the oxide semiconductor layer, oxygen in the oxide semiconductor layer is absorbed and lost in the process of oxidizing titanium or titanium alloys, thus resulting in difficulty in obtaining preferable characteristics.
It is difficult to sufficiently prevent the degradation of the TFT due to light entrance toward the semiconductor layer, in accordance with the techniques of Japanese Unexamined Patent Publications No. 2010-161373 and No. 2010-283326.